EN
JavaScript - Math.acos() method example
8
points
The Math.acos function returns number in radians in the range 0 to Math.PI. Based on this function we are able to calculate inverted cosine function value.
// ONLINE-RUNNER:browser;
// radians degrees
console.log( Math.acos( -2 ) ); // NaN
console.log( Math.acos( -1 ) ); // 3.1415926535897930 -> 180
console.log( Math.acos( -0.5 ) ); // 2.0943951023931957 -> ~120
console.log( Math.acos( 0 ) ); // 1.5707963267948966 -> 90
console.log( Math.acos( 0.5 ) ); // 1.0471975511965979 -> ~60
console.log( Math.acos( 1 ) ); // 0 -> 0
console.log( Math.acos( 2 ) ); // NaN
console.log( Math.acos( 0.7071067811865476 ) ); // 0.7853981633974483 -> 45 deg
console.log( Math.acos( 0.8660254037844387 ) ); // 0.5235987755982987 -> 30 deg
console.log(Math.acos( Math.cos( Math.PI / 4))); // 0.7853981633974483-> pi/4 (45deg)
console.log(Math.cos( Math.acos( Math.PI / 4))); // 0.7853981633974483-> pi/4 (45deg)
Another way to look at acos function:
// ONLINE-RUNNER:browser;
function calculateAngle(b, h) {
return Math.acos(b / h);
}
/*
|\
| \ h
a | \
|__*\ <- angle
b
*/
var a, b, h;
// a, b and h build isosceles right triangle
a = 3;
b = a;
h = Math.sqrt(a * a + b * b);
console.log( calculateAngle(b, h) ); // 0.7853981633974483 <- 45 degrees
// a, b and h build half of equilateral triangle
a = 3;
b = a * Math.sqrt(3);
h = Math.sqrt(a * a + b * b);
console.log( calculateAngle(b, h) ); // 0.5235987755982987 <- ~30 degrees
// a, b and h are not able to build triangle
a = 3;
b = a;
h = 1;
console.log( calculateAngle(b, h) ); // NaN
1. Documentation
| Syntax | Math.acos(number) |
| Parameters |
|
| Result |
If value can not be calculated |
| Description |
|
2. Working with degrees
// ONLINE-RUNNER:browser;
function calculateAngle(b, h) {
var angle = Math.acos(b / h);
return (180 / Math.PI) * angle; // rad to deg conversion
}
/*
|\
| \ h
a | \
|__*\ <- angle
b
*/
var a, b, h;
// a, b and h build isosceles right triangle
a = 3;
b = a;
h = Math.sqrt(a * a + b * b);
console.log( calculateAngle(b, h) ); // 45 degrees
// a, b and h build half of equilateral triangle
a = 3;
b = a * Math.sqrt(3);
h = Math.sqrt(a * a + b * b);
console.log( calculateAngle(b, h) ); // ~30 degrees
// a, b and h are not able to build triangle
a = 3;
b = a;
h = 1;
console.log( calculateAngle(b, h) ); // NaN
3. Canvas plot example
// ONLINE-RUNNER:browser;
<!doctype html>
<html>
<head>
<style> #canvas { border: 1px solid black; } </style>
</head>
<body>
<canvas id="canvas" width="200" height="200"></canvas>
<script>
var canvas = document.querySelector('#canvas');
var context = canvas.getContext('2d');
// arccosine chart range
var x1 = -1.0;
var x2 = +1.0;
var y1 = 0;
var y2 = +Math.PI
var dx = 0.01;
var xRange = x2 - x1;
var yRange = y2 - y1;
function calculatePoint(x) {
var y = Math.acos(x);
// chart will be reversed horizontaly because of reversed canvas pixels
var nx = (x - x1) / xRange; // normalized x
var ny = 1.0 - (y - y1) / yRange; // normalized y
var point = {
x: nx * canvas.width,
y: ny * canvas.height
};
return point;
}
console.log('x range: <' + x1 + '; ' + x2 + '>');
console.log('y range: <' + y1 + '; ' + y2 + '> // angles in radians');
var point = calculatePoint(x1);
context.beginPath();
context.moveTo(point.x, point.y);
for (var x = x1 + dx; x < x2; x += dx) {
point = calculatePoint(x);
context.lineTo(point.x, point.y);
}
point = calculatePoint(x2);
context.lineTo(point.x, point.y);
context.stroke();
</script>
</body>
</html>